1. Field of the Invention
The present invention relates to a silicon micro sensor available to an infrared ray sensor, flow sensor, gas sensor, pressure sensor, vibration sensor, acceleration sensor or the like which is manufactured using an anisotropic etching method for machining a silicon substrate.
2. Description of Related Art
In an infrared ray sensor, flow sensor or gas sensor utilizing heat transfer, heater and detection elements thereof are made finer and as thin as possible since a higher sensitivity and responsibility and a lower consumption of electric power can be obtained by minimizing a heat compacitance of the heater element and the detection element.
Besides, in a sensor having a vibrating element such as a pressure sensor, a vibration sensor or an acceleration sensor, it becomes possible to minimize the dimensions of the sensor by thinning the vibration element and a support element for supporting the same and, thereby, the sensor is able to show a high sensitivity since the vibration element is vibrated by a minor force such as pressure.
Further, in all of these sensors, a high sensitivity, minimization of the size and hybridation or integration of plural sensors is possible by forming the sensing element on a support member formed with a thin film.
Due to these reasons, silicon micro sensors, wherein support elements are formed by thin films, have been developed in recent years using so called micro machining techniques wherein the photo-lithograpy techniques are applied for etching silicon elements into fine structures utilizing the crystal anisotropic property.
Meanwhile, there are many types of support elements for supporting a sensor element such as a bridge type, cantilever type and diaphragm type as shown in FIGS. 3, 4 and 5, respectively.
Upon manufacturing the silicon micro sensors of these types, silicon oxide films 111, 112, and 113 are formed on respective silicon substrates 101, 102 and 103 using the thermal oxidization method or the CVD method at first. Then, each of silicon oxide films 111 to 113 is patterned to form a mask and each of the silicon substrates 101 to 103 is etched anisotropically utilizing the anisotropic property thereof. Thus, support elements 121, 122 and 123 of a bridge type, cantilever type and diaphragm type are formed as portions of respective silicon oxide films 111, 112 and 113 used as a mask, as clearly shown in FIGS. 3(b), 4(b) and 5(b).
However, upon forming the silicon oxide film using the thermal oxidization method or the CVD method, the surface of the silicon substrate is heated up to a temperature of 600.degree. to 1,000.degree. C. At such a high temperature, thermal stress is caused in the silicon oxide film due to the difference between coefficients of thermal expansion of the silicon substrate and the silicon oxide film and, thereby, cracks and breaks are likely to be caused in the silicon oxide film. Accordingly, it is difficult to form the support film with the silicon oxide film alone.
In order to solve this problem, the present inventors proposed to use a silicon oxide film or a silicon nitride film formed by the sputtering method as the support film. However, in this case, the support film is bent slightly due to the residual stress remaining therein. The bend of the support film is impossible to remove by the thermal treatment in the case of the silicon oxide film, however, in the case of the silicon nitride film, it is possible to remove the bend by processing the support film thermally at a temperature of 800.degree. C. or more. In the latter case, many problems are caused by the thermal treatment at such a high temperature.
On the other hand, it may be considered to use an aluminum oxide film formed as the support film by the sputtering method. Although the film is slightly bent also in the case of the aluminum oxide film, the bend thereof can be removed by the thermal treatment at a relatively low temperature about 500.degree. C. The reason why it is possible at such a low temperature is that the aluminum oxide has a coefficient of thermal expansion greater than that of the silicon nitride. However, many defects due to contaminants such as dusts are contained in the aluminum oxide film and, therefore, upon etching the silicon substrate, the etchant penetrating through these defects causes etch pits therein.